A thermostable secreted protease denoted PfuS was isolated from Pyrococcus furiosus and has previously been produced using an expression DNA construct including a secretion signal but yields of the PfuS protease were rather low (EP0994191A1; Takara, JP). It is of interest to increase the expression yields of the thermostabile PfuS protease from Pyrococcus furiosus, so that it can be produced in sufficient quantities and at an acceptable production economy in order for the enzyme to be employed industrially.
A thermostable secreted xylanase denoted XynB was identified and cloned from Dictyoglomus thermophilum, the enzyme was recombinantly expressed in E. coli. The mature N terminus of XynB was located downstream of a 23-amino-acid leader peptide, as predicted by the SignalP signal peptide analysis software and supported by the results of a multiple sequence analysis performed with other bacterial family 11 xylanases. The XynB leader peptide made the enzyme toxic for E. coli. The XynB enzyme was expressed in E. coli without leader peptide and recovered by cell lysis (Morris D D et al. 1998, Appl. Environ. Microbiol. 64 (5): 1759-65). The XynB enzyme was also expressed recombinantly in a Bacillus subtilis host with its native secretion signal but only in very low yields (Zhang et al; Appl Biochem Biotechnol (2010) 160:1484-1495). It is of interest to increase the expression yields of the thermostabile XynB xylanase from Dictyoglomus thermophilum, so that it can be produced in sufficient quantities and at an acceptable production economy in order for the enzyme to be employed industrially.
Bacillus host cells have been characterized as workhorses in the industrial manufacture of various polypeptides of interest, mainly because they actively secrete polypeptides that have a so-called signal peptide. The signal peptide is a small polypeptide, typically around 20-30 amino acids, that is expressed in transcriptional and translational fusion with the N-terminal of a polypeptide to be secreted. It directs the fused polypeptide into the secretory machinery of a suitably equipped host cell, whereupon the fused polypeptide is cleaved while the now-matured polypeptide of interest is secreted into the surrounding culture broth without its signal peptide which is retained in the cell and degraded.
The secretion of recombinantly produced polypeptides of interest in Bacillus enables a comparatively easy recovery of the polypeptides directly from the culture broth without having to perform a cell lysis step. Only polypeptides destined to be exported from the cell into the growth medium are natively outfitted with a signal peptide in Bacillus. 
Most polypeptides in a cell are not destined for export but are instead intended to be intracellular, periplasmic, membrane-bound etc. Such natively non-secreted polypeptides are usually considered burdensome to produce because they either need to be recovered from within the host cells which usually requires a messy cell-lysis step resulting in a challenging recovery process which, in turn, is why natively secreted enzymes are preferred for industrial manufacture.